It is known that circuit breakers and disconnectors, hereinafter referred to as a whole as switches, comprise an outer casing and one or more electrical poles to each of which are associated at least one fixed contact and at least one mobile contact that can be coupled to/uncoupled from one another.
Circuit breakers of the known art moreover comprise control means that enable displacement of the mobile contacts, causing their coupling to or uncoupling from the corresponding fixed contacts. The action of said control means is traditionally exerted on a main shaft that is operatively connected to the mobile contacts so that, following upon its rotation, the mobile contacts will be brought from a first operative position to a second operative position, which are respectively characteristic of a configuration of switch open and of switch closed.
In the case of switches for low currents, indicatively up to 800 A, there exist solutions that cause the main shaft to coincide with the mobile contacts, giving rise to a rotating element made of insulating material capable of guaranteeing both dielectric separation between the phases and, of course, proper transmission of the movements and resistance to the forces involved. The rotating element is usually supported by structural parts of the outer casing of the switch, which basically define areas of bearing with the rotating element itself. Switches of this type present considerable advantages, such as, for example, a limited number of parts and a limited overall encumbrance.
The indicative technical limit of 800 A for the switches that make use of the rotating element derive from the fact that, beyond this limit, there would be required of the rotating element performance of mechanical resistance that is scarcely compatible with structural materials of an insulating type that are to have competitive costs.
From a practical standpoint, the requirement of higher mechanical characteristics has partially been met by introducing metal reinforcement bars, passing through the rotating element itself. The metal reinforcement bars pose, however, problems of interference with the characteristics of electrical insulation between the poles. In practice, only modest increases of performance are obtained with costly and industrially complex solutions.
Another road followed in the known art for bestowing upon the rotating element higher mechanical characteristics is that of increasing the radial dimensions thereof; solutions of this second type tend, however, to introduce greater friction and jeopardize the general efficiency of the switch.
A more advanced solution, described in the patent application No. BG2005A000026 enables extension of the use of the rotating element also to switches for currents decidedly higher than 800 A by introducing bearings that suspend the rotating element itself from the control members. In particular, the latter solution reduces the friction and prevents the stresses from being transmitted by the contacts to the rotating element directly onto critical areas of the switch, such as, for example, the joints of the containment means.
Even though the latter solution enables exploitation of the switch over a particularly extensive range of performance levels, there remain in any case physical limits of use linked not so much to the rated current as rather to the electrodynamic strength and to the breaking power of the switch. A good electrodynamic strength would require in fact the use of particularly strong contact springs, whilst the breaking power of the switch is linked, among other things, to the capacity of the rotating element to absorb without damage the mechanical stresses transmitted by the contacts following upon electrodynamic repulsion. In practice, these limits are substantially dictated by the resistance of the joints between the pins of the individual poles and the rotating element itself. The design data must in fact guarantee that the plastic material that makes up the rotating element works exclusively in the so-called region of elastic behaviour. Once said limit is exceeded, the so-called phenomena of yielding and failure would in fact start.
It may be readily noted how this limit is relatively modest even with the use of high-quality plastic materials, such as, for example, the so-called moulding compounds with a base of unsaturated polyester.
Since the electrodynamic strength and the electrodynamic repulsion of the mobile contacts cause considerable stresses, above all of thrust and tugging, in the area of the rotating element in which the pins are fixed, it is clear that wishing to achieve further increased performance for the switch it is necessary to increase the resistance to stresses of the rotating element, guaranteeing at the same time the electrical insulation between the phases.